Abstract
Cyclo (-Tyr-Arg-) (I) and its three stereoisomers, in which one (or both) of the constitutive amino acids was replaced by the corresponding D-amino acid, were synthesized. Diketopiperazines were prepared from dipeptide methyl esters by the use of acetic acid as a catalyst. When administered intracerebrally into mice, I exhibited more potent analgesia than its three stereoisomers, and its activity was five times more potent than that of H-Tyr-Arg-OH. The positions of the two side chains of diketopiperazines are discussed on the basis of the proton magnetic resonance spectral data.
